The widespread use of the Internet has led to a rapid increase in traffic volume for backbone communication systems. This has created a desire for realization of practical optical communication systems operating at ultra-high speed exceeding 100 Gbps. One technology attracting attention to realize ultrafast optical communication systems is the digital coherent scheme that combines an optical phase modulation scheme with a polarization multiplexing and demultiplexing technique.
PTL 1 and NPL 1 respectively disclose techniques to compensate for a frequency deviation in digital coherent receivers.
The digital coherent receiver described in NPL 1 uses local oscillation light whose oscillating frequency can be controlled to compensate for a frequency deviation by controlling the oscillating frequency of the local oscillation light in the direction opposite to a frequency deviation setting value. However, the configuration described in NPL 1 requires an arrangement for controlling the oscillating frequency of the local oscillation light.
The digital coherent receiver according to PTL 1 compensates for waveform distortion by conducting an overlap-type fast Fourier transform (FFT) and inverse FFT (IFFT). This digital coherent receiver has circuitry which includes an input unit, an FFT input frame generation unit, an FFT processing unit, a characteristic multiplication unit, an IFFT processing unit, an IFFT output frame extraction unit, and an output unit. Assuming that the input data consists of 256 parallel signals and the window size for FFT and IFFT is 1,024, the digital coherent receiver according to PTL 1 operates as follows.
The input unit buffers the input data (time domain: 256 samples), generates a frame consisting of 512 samples every two clocks, and outputs the frame to the FFT input frame generation unit.
With respect to the inputted sample frames, the FFT input frame generation unit generates a frame consisting of 1,024 samples by combining the current 512-sample frame with the immediately preceding 512-sample frame and outputs the generated frame to the FFT processing unit.
The FFT processing unit transforms the inputted frame into frequency-domain data and outputs it to the characteristic multiplication unit.
With respect to the inputted frequency-domain data, the characteristic multiplication unit multiplies characteristic parameters for each frequency component (1,024 frequencies) and outputs the results to the IFFT processing unit. The characteristic parameters are inputted, for example, from an external area.
The IFFT processing unit transforms the inputted frame into time-domain data and outputs it to the IFFT output frame extraction unit. In front and behind of this frame outputted from the IFFT processing unit, discontinuous points are included.
The IFFT output frame extraction unit discards 256 samples each, i.e., a quarter of the window size, from the front and rear of the inputted frame. If discontinuous points are within the area discarded by the IFFT output frame extraction unit, no discontinuous points are generated in the output obtained by joining 512 samples that have not been discarded. The IFFT output frame extraction unit outputs the processed frame to the output unit.
The output unit takes out every 256 samples per one clock from an inputted frame (512 samples outputted every two clocks) and outputs them to the subsequent stage in the form of parallel signals.
The above-described digital coherent receiver according to PTL 1 includes circuitry for performing the above-mentioned overlap-type FFT and IFFT to inhibit discontinuous points from occurring.